Engine coolants for the cooling system of an automotive vehicle generally contain ethylene glycol, alone or with a small percentage of diethylene glycol, and a suitable corrosion inhibitor. These inhibitors are usually a mixture of one or more inorganic salts, such as phosphates, borates, nitrates, nitrites, silicates or arsenates, and an organic compound, such as benzotriazole, tolyltriazole or mercaptobenzothiozole, to prevent copper corrosion. Similar inhibitors would be utilized where aluminum corrosion could be a problem. The solution is generally buffered to a pH of 8 to 10 to reduce iron corrosion and to neutralize any glycolic acid formed in the oxidation of ethylene glycol.
Over a period of time, the corrosion inhibitor in the coolant may be lost or at least decreased in concentration due to leakage, hose breakage or boil over, or the inhibitor may decrease in effectiveness due to age. If the corrosion inhibitor in the coolant decreases, metal corrosion will increase significantly. This is especially true for higher temperature coolant systems or where new lightweight aluminum radiators are substituted for conventional copper brass radiators.
In the copending U.S. patent application Ser. No. 88,506 filed Oct. 26, 1979, a container is disclosed which was suitably secured in a coolant line to the radiator with a corrodible end wall exposed to the coolant flowing through the line so that, if the coolant became corrosive, the end of the container would corrode through to release corrosion inhibitor in the container into the coolant stream to reduce the corrosiveness of the coolant before corrosion of the radiator became a problem. For an aluminum radiator, the end wall of the container was formed of aluminum or an aluminum alloy, and the wall surface exposed to the coolant was scored or knurled to enhance localized corrosion.
However, although the end surface of the container will pit and corrode to allow liquid to enter and dissolve the corrosion inhibitor prior to serious corrosion of the radiator or other components of the cooling system, it would be desirable to speed up the corrosion process of the container surface to shorten the time interval between the coolant reaching the predetermined corrosive level and the point when the corrosion inhibitor is effectively released into the coolant. The present invention provides a container membrane which will act to shorten that time interval.